Water Demand Calculation : To Design Overhead Water Tank

 Water Demand and its Variation

Over Head Tank

Water demand

Water is a prime necessity for life and that has led to growth of population along the banks of natural water courses and springs. The water required for various purposes as domestic, livestock, institutional and commercial, fire fighting, industrial, municipal or public, losses or wastage demand are the types of water demand. The amount of water required for a rural/urban community depends on factors like the economic level of community, their consciousness and other physical and social aspects. This various demand is not essential to take in account to calculate total water demand of city or community or town so that firefighting, municipal, industrial demand may be excluded in rural areas where as livestock demand is excluded in urban area.

 

1. Domestic Water demand:

 

This includes the water which is required for use in private residence for drinking, cooking, and bathing, washing of cloths, lawn watering and gardening and sanitary purpose. The amount of domestic water demand depends on the living conditions of the consumers, · climatic condition, habit, social status etc. The design of gravity flow community water supply schemes in Nepal, in the past has taken 45 lpcd as per the recommendation of the WHO. The provision of 45lpcd is considered to include allowances for drinking and cooking, personal washing, wastage and leakage, and some portion of the domestic animal demands.

Generally adopted per capita domestic needs are as follows: -

2 Livestock Water demand:

The quantity of water required for domestic animals is called livestock demand. Livestock is an important component of the life style in rural Nepal. It has utilizes both as draft animals for tilling land and a source of income. The water consumption by the livestock (animals) must be known specially for farms and livestock areas. The approximate consumption given below:-

Big animal (Cowl buffalo/horse) 45 lit/animal

Small animal(Goat, dog, rabbit etc.) 20 lit/animal

Poultry(Birds, chicken, duck etc.) 0.2 lit/birds

3 Commercial /Institutional demand Water demand:

 It includes the demand for commercial establishments and institutions like universities, school, cinema hall, office building, warehouse, stores, hotels, hospitals theaters, clubs etc. Institutional demand refers to the water needed for offices, schools and health posts, in the community. In some case, tourist resorts, local industries may also have to be supplied water from the scheme. Government institutions deriving service from the water scheme must also support the water user committee (WUC) in operating and maintenance the schemes. In community water supply scheme priority should be given to supply water to

schools and health posts. The following institutional water demand should be adopted:-

 

4. Public/Municipal Water demand:

Water required for public or municipal utility such as washing and sprinkling on road, flushing sewers, watering public parks . etc. is municipal or public demand. A provision of 5 to 10% of the total demand is taken as this demand. This demand is only considered in urban water supply system.

5. Industrial demand Water demand:

Industrial area could be located far from the city though it may locate in periphery of city which may be vital in calculating water demand. Normally 20-25% of total demand is taken for industrial demand. It is considered only in urban area and depends upon the type and size of industry.

6. Fire fighting Water demand:

Fire is generally break in thickly populated localities and the industrial area and causes serious damages of properties. During outbreak of fire, the water is used for firefighting is called fire demand. This demand is not fixed so it is difficult to calculate demand. Different empirical formula can be used to determine fire demand but it cannot directly used for community context. This demand is considered in urban water supply system.

The quantity of water required for fire fighting is calculated by following formula

If Q= water required in l/min and 

P= population in thousands then.

(a) According to National board of fire

Q = 4637  ÖP (1-0.01 ÖP )

Where Q=quantity in lit/sec

            P = population in thousands

(b) According to Freeman’s formula

Q = 1136.50(P/5 + 10)

Where Q=quantity in lit/sec

P = population in thousands

(c) Kuichling’s formula,

Q = 3182 ÖP

Where Q=quantity in lit/sec

P = population in thousands 

(d) Buston’s formula

Q = 5663 ÖP

 Where Q=quantity in lit/sec

P = population in thousands

(e) Indian water supply manual (1976) formula:

Q = 100 ÖP where Q in m3/day

Where Q=quantity in lit/sec

P = population in thousands 

7. Loss and wastage

 Loss and wastage may be termed as unaccounted for water which includes

water due to faulty valves and fittings, poor distribution system, defective pipes, unauthorized

connections, tap open etc. Lost and wasted being uncertain it cannot be predicted precisely so

generally it is taken as 15 to 20% of total demand.

Causes of loose and wastage of water.

* Leakage and over flow reservoir.

* Leakage from main and service pipe connections.

* Leakage and looses from consumers premises.

* Wastage for public taps. 

8. Total water demand

The sum of all water demands is total water demand as given below.

TD=DD+LD+ID+ID+PD+FD+LD

Where,

TD = Total water demand

DD = Domestic water demand

LD = livestock demand

ID = institutional and commercial demand

ID = Industrial demand

PD = Public/municipal demand

FD = fire demand

LD = Losses and wastage demand

9. Per capita Water demand:

 It represents the average consumption or demand of water per day for various purposes including all demand of water for person. It is expressed as Lpcd/Lphd.

Let 'Q' be the total quantity of water required per year in liters by city or town having population (P), and per capita demand or rate of water demand (q) usually expressed in lpcd is given by the following expression .

 

Water demand variation.

The rate of demand of water represents the average consumption or demand of water per capita/head per day. Rate of demand does not remain constant but varies with the season or month of the year, with the days of week, and with the hours of the day. These variations in the rate of demand of water are termed as,

i)               Seasonal variations 

The rate of demand of water varies considerably from season to season. In summer water demands usually 30 to 40% above the annual average rate of flow of water, because more water is required for drinking, bathing, washing etc. In winter the average rate of demand is about 20% lower than the annual average rate of demand of water because of less requirement of water.

Q_seasonal = 1.3 X Q_average (In India)

 

ii)             Daily variations 

Due to change in the day to day climatic conditions, or due to the day being a holiday or some

festivals day the rate of demand of water varies from day to day and called daily variations.

 

Q_daily = 1.8 X Q_average (In India)